The D vitamins are very important agents for the control of calcium and phosphate metabolism in animals and humans, and have long been used as dietary supplements and in clinical practice to assure proper bone growth and development. It is now know that the in vivo activity of these vitamins, specifically of vitamin D.sub.2 and D.sub.3, is dependent on metabolism to hydroxylated forms. Thus, vitamin D.sub.3 undergoes two successive hydroxylation reactions in vivo, leading first to 25-hydroxyvitamin D.sub.3 and then to 1,25-dihydroxyvitamin D.sub.3 and the latter is thought to be the compound responsible for the well-known beneficial effects of vitamin D.sub.3. Likewise, vitamin D.sub.2, which is commonly used as a dietary supplement, undergoes an analogous hydroxylation sequence to its active forms, being first converted to 25-hydroxyvitamin D.sub.2 (25-OH-D.sub.2) and then to 1,25-dihydroxyvitamin D.sub.2 (1,25-(OH).sub.2 D.sub.2). These facts are well established and well known in the art [see, for example, Suda et al. Biochemistry 8, 3515 (1969) and Jones et al. Biochemistry 14, 1250 (1975)].
Like the metabolites of the vitamin D.sub.3 series, the hydroxylated forms of vitamin D.sub.2 named above are, because of their potency and other beneficial properties, highly desirable dietary supplements, or pharmaceutical agents, for the cure or prevention of bone or related diseases, and their value and possible use is recognized in patents relating to these compounds [U.S. Pat. Nos. 3,585,221 and 3,880,894].
Whereas many metabolites of vitamin D.sub.3 have been prepared by chemical synthesis, there has been less work on the preparation of vitamin D.sub.2 metabolites. The known synthetic processes for the metabolites of the D.sub.3 -series (especially as far as they relate to the preparation of side chain hydroxylated compounds) are, of course, in general not suitable for the preparation of the corresponding vitamin D.sub.2 metabolites, since the latter are characterized by a side chain structure (i.e. presence of a double bond and an extra methyl group) which requires a different synthetic approach from that applicable to side chain hydroxylated D.sub.3 compounds.
Various approaches for the preparation of vitamin D.sub.2 metabolites are known, and are described in U.S. Pat. Nos. 4,448,721, 4,847,012 and 4,769,181. Other preparations of 25-OH-D.sub.2 and 1,25-(OH).sub.2 D.sub.2 compounds involving condensation of side chains with a steroid nucleus are shown in Yamada et al, "Facile And Stereoselective Synthesis of 25-Hydroxyvitamin D.sub.2 ", Tetrahedron Letters, Vol. 25, No. 33, pp. 3347-3350, 1984 and in Tsuji et al, "A New And Convenient Synthesis of 1.alpha., 25-Dihydroxyvitamin D.sub.2 And Its 24R-Epimer," Bull. Chem. Soc. Jpn., Vol. 62, No. 10, pp. 3132-3137, 1989. Perlman et al have reported the preparation of the epimer of 1.alpha.-OH-D.sub.2 by condensation of a suitable side chain fragment with a vitamin D nucleus in J. Chem. Soc. Chem. Com. pp. 1113-1115, 1989.